>>Why not a court martial?<<
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An issue of jurisdiction. Had the murder occurred on a military facility, certainly a court martial. It occurred off base, so the civilian authorities will handle the case. The Marine Corps eventually will discharge him.
It doesn't matter.An active duty member of the Armed Forces is subject to the UCMJ,every minute of every day,regardless (don't know how it works for Reserves/Guard). If the Navy chose to do so he *could* be court martial-ed despite being "off duty" and "off base".
I suppose that the Navy might decline to prosecute as a courtesy to the State of Hawaii...but IMO a crime like this calls for much more that 20 years in state prison.